Field of the Invention
The present invention relates to a dielectric porcelain composition and dielectric element having such composition.
Description of the Related Art
Electronic components are used in various fields in recent years, and in some cases they are used in severe environments. For example, electronic components used for such applications as operation of power devices based on SiC or GaN that are drawing attention as onboard devices, noise elimination in the engine compartments of cars, etc., are required to have a specific dielectric constant with good temperature properties even at high temperatures such as 200 to 350° C. However, barium titanate, which is widely used as a dielectric porcelain composition to constitute capacitors, cannot meet the aforementioned requirement because its Curie temperature is near 130° C. and therefore its specific dielectric constant drops significantly in a temperature range of 150° C. or above.
Materials containing Pb have traditionally been used in such high-temperature applications. For example, a dielectric porcelain composition comprised of two components, such as one expressed by PbTiO3—BaZrO3, is such that its specific dielectric constant is relatively less dependent on temperature until around 300° C. because the Curie point of PbTiO3 is around 490° C. On the other hand, the specific dielectric constant of Pb(Zr0.95Ti0.05)O3 (PLZT), containing La, exhibits good bias dependence at high temperature because of the material's anti-ferroelectricity which is different from that of barium titanate mentioned above. These materials were invented in light of the problems presented by barium titanate, but Pb contained in such compositions is an environmentally hazardous substance whose use is not desired. Accordingly, there is a demand for Pb-free materials offering good properties even in a high temperature range.
Lead-free bismuth niobate piezoelectric porcelain compositions are known (see Patent Literatures 1 to 3, for example). However, these patent literatures do not disclose reducing the temperature dependence of the specific dielectric constant of the dielectric porcelain composition over a wide temperature range. Furthermore, in the inventions in Patent Literatures 2 and 3, the ceramic sintering temperature is high at 1050° C. or above.
When a lead-free bismuth niobate composition with a high Curie point and good temperature properties is considered as a material for the dielectric layers of a laminate capacitor, desirably silver palladium alloy is used for the internal electrodes. This simplifies the process because sintering can be done in an atmospheric ambience unlike when Ni electrodes are used. There is also a cost advantage because the sintering temperature is low. The cost and sintering temperature of a silver palladium alloy (Ag—Pd) electrode vary depending on its silver (Ag) and palladium (Pd) ratio (Ag/Pd), but desirably the Ag ratio (Ag/Pd) is 7/10 or higher when cost is considered.
Simultaneous sintering of bismuth niobate BNT-BT-KNN (Bi1/2Na1/2TiO3—BaTiO3—K0.5Na0.5NbO3) with Pd electrodes has been reported. As for Ag electrodes, a study by Nagata et al. (Non-patent Literature 1) shows that sintering BKT (Bi0.5K0.5TiO3) at 1050° C. causes Bi and Ag to react. Accordingly, sintering at temperatures below 1050° C. is desired for Ag—Pd electrodes offering cost advantage, in order to prevent reaction between Bi and Ag. This way, electrodes having an Ag/Pd ratio of 7/3, 8/2, 9/1, etc., also become feasible. Some kind of sintering auxiliary is needed to lower the sintering temperature. In Non-patent Literature 2, broadening of the dielectric constant with an increase in the KNN ratio of BNT-BT-KNN is reported. This means that the rate of change in capacitance relative to temperature change is small. However, increasing the KNN ratio causes the sintering temperature to rise and the relative density of the sintered body to drop, which is a problem.